The present invention relates to simultaneous spatial modulation of angularly multiplexed optical beams that are individually coherent but mutually incoherent, to multiplexed volume holographic recording, readout, and interconnections, and, more particularly, to photonic interconnections, photonic implementations of neural networks, optical signal processing, optical information processing and computing, optical memory, optical displays, copying of multiplexed volume holograms, and multiplexed volume holographic optical elements.
A wide variety of information systems applications exist that require a high density of interconnections among device or system nodes, or a high density of rapidly accessible memory, or both. These applications include, for example, neural networks, telecommunications switching systems, digital computing, optical displays, and information (including signal) processing. In many such applications, key requirements on the chosen interconnection technology include low insertion losses, high interchannel isolation (freedom from interchannel crosstalk), a high degree of potential fan-in and fan-out at each node, weighted interconnection channels, and high capacity. Comparable requirements exist for the chosen memory technology, including low latency (rapid information access), parallel information retrieval, low effective bit error rates (high signal-to-noise ratio), high density information storage, and input/output compatibility with the remainder of the system.
In order to satisfy these many and varied requirements, multiplexed volume holographic optical elements provide an attractive alternative to electronic implementations of high capacity interconnection and memory elements. In fact, the very nature of a volume holographic optical element tends to blur the distinction between a pure interconnection network on the one hand, and a pure memory subsystem on the other, as it is in many ways simultaneously well-suited to both roles. Even so, previous methods for recording information or interconnection patterns in highly multiplexed volume holographic optical elements, and for reading them out, have not proven satisfactory in terms of throughput, crosstalk, and capacity. Furthermore, they have not proven to be manufacturable, due to the fact that information from a master volume holographic optical element could not previously be efficiently transferred to or duplicated in another such element.
In forming multiplexed volume holograms, one of three approaches is typically taken: (1) sequential, which involves several temporally-sequenced (and hence incoherent) exposures of the individual components of the hologram, done by rotating or translating the hologram (or the source beam, reference beam, or object beam); (2) simultaneous and fully coherent, which involves the use of two or more mutually coherent beams, each encoded with information and serving as a reference beam for the other(s); and (3) some combination of sequential and simultaneous fully coherent.
The first approach has the major disadvantage that temporal sequencing is time-consumptive, which can be of considerable importance in applications envisioned herein, for which the number of independent interconnections that must be recorded is extremely large. Also, in many holographic recording materials, sequential exposures tend to erase previously recorded information, leading to the necessity of incorporating unwieldy programmed recording sequences in order to result in the storage of a predetermined set of interconnections.
The second approach is designed to circumvent the above sequencing difficulties, but suffers instead from the coherent recording of unwanted interference patterns (holograms) that give rise to deleterious crosstalk among the various (supposedly independent) reconstructions, as described in more detail below.
The third approach is subject both to sequential recording time delays and the necessity for programmed recording schedules, as well as to the generation of undesirable crosstalk. As such, none of the previously employed multiplexed recording techniques allows for the generation of three-dimensional, truly independent interconnections between two or more two-dimensional planar arrays within the context of a temporally efficient recording scheme.
In all of the prior art approaches to the holographic recording of a multiplexed interconnection, two primary forms of interchannel crosstalk are encountered to a greater or lesser extent. Coherent recording crosstalk arises from the simultaneous use of multiple object and reference beams, all mutually coherent with each other. The mutual coherence causes additional interconnections to be formed other than those desired. Reconstruction with independently valued inputs results in the generation of output beams that cross-couple through the undesired interconnection pathways, which compromises the independence of the desired interconnection channels.
A second, unrelated form of crosstalk arises due to beam degeneracy, which occurs whenever a single object beam is used with a set of reference beams to record a fan-in interconnection to a single output node (e.g., neuron unit in the case of the photonic implementation of neural networks). (Fan-in is the connection of multiple interconnection lines to a common output node.) This latter form of crosstalk is present even when the set of object beams is recorded sequentially.
Of at least equally serious consequence is the optical throughput loss that results from interconnection fan-in so constructed as to exhibit beam degeneracy. In many well-documented cases, this loss is severe, resulting in at least an (Nxe2x88x921)/N loss (or, equivalently, a 1/N throughput efficiency) for the case of an N-input, N-output interconnection system, as reported by J. W. Goodman, Optica Acta, Vol. 32, pages 1489-1496 (1985). This is a truly daunting loss factor for interconnection systems such as those envisioned for neural networks, which may both require and be capable of 105 to 106 inputs and outputs.
In certain types of photorefractive materials, an additional throughput loss can arise from the incoherent superposition of several gratings within the same volume of the holographic optical element, due to the reduction in the effective modulation depth of the recorded holographic fringes. This effect occurs primarily in photorefractive crystals that generate an index of refraction or absorption change in response to local gradients in the intensity distribution, but would not be expected to occur in linear photorefractive materials that generate an index of refraction or absorption change in direct proportion to the magnitude of the local intensity distribution. In a number of cases, this effect can also result in at least an (Nxe2x88x921)/N loss for the case of an N-input, N-output interconnection system, as reported by P. Asthana, xe2x80x9cVolume Holographic Techniques for Highly Multiplexed Interconnection Applicationsxe2x80x9d, Ph.D. Dissertation, University of Southern California (1991), available from University Microfilms, Ann Arbor, Mich.
In the prior art, few attempts have been made to address the extremely important technological problem of duplicating the contents of a fully recorded, heavily multiplexed volume holographic optical element or interconnection device, particularly in the case of neural network interconnections. For example, to the inventors"" knowledge, there is no known prior technique for rapid copying of a volume hologram that is angularly multiplexed in two dimensions, other than that described in the parent and grandparent applications of the present application.
In the case of neural network interconnections, the training and/or learning sequences may be quite involved; in some cases, the training and/or learning sequences may result in a unique interconnection, and the exact learning sequence may not be reproducible in and of itself at all. In such cases, it is desirable to replicate the contents of the interconnection medium in such a manner that a fully functional copy is produced, as characterized by a complete operational set of interconnections indistinguishable from those implemented by the master. The method of replication must not demand an extremely lengthy recording sequence, must not be inefficient in its utilization of the programmed recording schedule and/or the total optical energy available for reproduction purposes, and must not induce additional optical throughput loss or interchannel cross-talk beyond that already incorporated in the master.
In the grandparent application of the present application, xe2x80x9cIncoherent/Coherent Multiplexed Holographic Recording for Photonic Interconnections and Holographic Optical Elementsxe2x80x9d, now issued as U.S. Pat. No. 5,121,231, (Jun. 9, 1992), apparatus for the incoherent/coherent multiplexed holographic recording of photonic interconnections and holographic optical elements is described. Specifically, apparatus for providing multiplexed volume holographic recording and readout comprises:
(a) means for providing an array of coherent light sources that are mutually incoherent;
(b) means for simultaneously forming an object beam and a reference beam from each coherent light source, thereby forming a set of multiplexed object beams and a separate set of multiplexed reference beams;
(c) means for either independently modulating each object beam, or spatially modulating a set of object beams so that all object beams are identically modulated;
(d) means for either independently modulating each reference beam, or spatially modulating a set of reference beams so that all reference beams are identically modulated;
(e) a holographic medium capable of simultaneously recording therein a holographic interference pattern produced by at least a portion of the set of all modulated multiplexed object beams and of the set of all modulated multiplexed reference beams pairwise, with all such pairs being mutually incoherent with respect to one another; and
(f) means for directing at least a portion of the set of modulated object beams and of the set of modulated reference beams onto the holographic medium and for interfering the portion of the modulated object beams and of the set of modulated reference beams, pairwise, inside the holographic medium.
Although the primary mode of multiplexing is angular, spatial and/or wavelength multiplexing may also be incorporated.
The architecture and apparatus described in the grandparent application significantly reduce coherent recording cross-talk and beam degeneracy crosstalk, and permit simultaneous network initiation, simultaneous weight updates, and incoherent summing at each output node without significant fan-in loss.
Further in accordance with the grandparent application of the present application, the above apparatus is provided with means for controllably blocking the set of object beams such that at least a portion of the set of reference beams (either modulated or unmodulated) reconstruct a stored holographic interference pattern. In one embodiment, the reconstructed pattern is angularly multiplexed and detected in such a manner as to produce an incoherent summation on a pixel-by-pixel basis of the reconstructed set of object beams. In this manner, multiplexed volume holographic recording and readout are provided.
Specific implementations to neural networks, telecommunication interconnections (e.g., local area networks and long distance switching), interconnections for digital computing, and multiplexed holographic optical elements are provided in both the grandparent and the parent applications.
In addition, apparatus for copying a multiplexed volume hologram is provided in the grandparent application. The apparatus comprises:
(a) means for providing an array of coherent light sources that are mutually incoherent;
(b) means for forming two reference beams from each coherent light source, thereby forming two sets of multiplexed reference beams, each set at a different location;
(c) means for directing the first set of reference beams onto the original multiplexed volume hologram to thereby form a set of output beams;
(d) means for directing the second set of reference beams onto a secondary holographic recording medium;
(e) means for directing the set of output beams from the original multiplexed volume hologram onto the secondary holographic recording medium, with path lengths sufficiently identical to the reference beam path lengths to permit coherent interference, pairwise, between the output beams and the second set of reference beams, inside the secondary holographic recording medium; and
(f) means for simultaneously recording in the secondary holographic medium a holographic interference pattern produced by the set of output beams and the second set of reference beams, thereby forming the substantially identical multiplexed volume hologram.
Portions of the above-described apparatus also possess unique properties and give rise to useful functions. It is to these unique portions, or elements, that the present application is directed.
In addition, specific implementations are given that exploit the double angular multiplexing features of the apparatus described in both the grandparent and the parent applications.
In accordance with the invention, apparatus is provided for simultaneous spatial modulation of angularly multiplexed individually coherent but mutually incoherent beams, comprising:
(a) means for generating a set of two or more individually coherent beams;
(b) means for simultaneously activating at least the set of individually coherent beams, which have at least one optical wavelength in common, pairwise;
(c) means for assuring mutual incoherence among the individually coherent beams, thereby providing a set of two or more individually coherent but mutually incoherent beams; and
(d) means for directing the set of individually coherent but mutually incoherent beams to a spatial modulation means for spatially modulating the set of beams, such that:
(i) a spatially overlapping group of beams, comprising at least two of the individually coherent but mutually incoherent beams, overlap spatially in at least one region of the spatial modulation means, thereby providing a spatially modulated set of beam, and
(ii) the spatially overlapping group of beams is angularly multiplexed within at least one such region.
Further in accordance with the invention, apparatus is provided for simultaneous spatial modulation of angularly multiplexed individually coherent but mutually incoherent beams, comprising:
(a) an incoherent/coherent source array, further comprising:
(i) a set of individually coherent light sources within a two-dimensional source array,
(ii) means for simultaneously activating at least a group of two or more of the individually coherent light sources, with the group of two or more individually coherent light sources thereby generating a group of two or more individually coherent beams that are substantially monochromatic, and
(iii) means for assuring mutual incoherence among substantially all of the individually coherent beams, pairwise, thereby providing a group of two or more individually coherent but mutually incoherent beams; and
(b) means for directing the group of two or more individually coherent but mutually incoherent beams to a spatial modulation means for spatially modulating the group of beams, such that:
(i) a spatially overlapping subgroup of beams, comprising at least two of the individually coherent but mutually incoherent beams, overlap spatially in at least one region of the spatial modulation means, thereby providing a spatially modulated set of beams, and
(ii) the spatially overlapping subgroup of beams is angularly multiplexed within at least one such region.
Variants of the apparatus are provided that allow for the incorporation of optically isolated semiconductor laser diode arrays (both edge-emitting and surface-emitting), vertical cavity surface emitting laser arrays that are partially mutually incoherent and partially wavelength division multiplexed, the combination of laser diode and phase modulator arrays to provide for controllable mutual incoherence, both pixelated and non-pixelated spatial light modulators, both full and partial overlap of beams at the spatial modulation means, and the incorporation of both one- and two-dimensional detector arrays.
Provision is further given for the spatial modulation means to comprise at least one of a spatial light modulator, a planar hologram, a planar holographic optical element, a planar diffractive optical element, a volume hologram, a volume holographic optical element, and a volume diffractive optical element.
Specific implementations are provided to: optical interconnections in neural networks, digital computing, and telecommunications; holographic optical elements; optical information processing; and optical memory. Further application to optical displays is made possible by the teachings of the invention.